1. Field of Art
Magnesium oxychloride cement has been used as a bonding agent since early in the twentieth century. While processes for preparing magnesium oxychloride cement bonds are well-known in the art, the chemistry of these processes is not fully understood to this day. The cement is a complex reaction product of calcined magnesium oxide, magnesium chloride, and water, which is admixed in standard proportions and allowed to cure or harden into a cement variously designated in the industry as "magnesite cement", "magnetic cement", "Sorel cement", "French cement", and other terms. "Magnesium oxychloride cement bond" and "magnesite bond" are used interchangeably herein.
Curable magnesium oxychloride cement compositions broadly range from castable mixes to mixes that are quite stiff and are tamped into molds. While the invention herein is applicable to compositions encompassing at least this broad range of formulations, it is particularly applicable to the latter compositions, those that are stiff, and tamped into a mold such as a wheel mold, wherein they are cured to provide the desired shaped product.
Magnesium oxychloride cement has particular application in the production of abrasive tools used for grinding, especially abrasive wheels. The wheels are typically prepared by admixing the bond precursors with abrasive grit and optional additives, molding the mixture into a wheel shape by the above-described tamping operation, and curing the mixture to harden the bond components into a magnesium oxychloride cement bond containing abrasive grit material of a type and amount suitable for the contemplated grinding application.
The product has grinding characteristics eminently suitable for dry grinding applications. Unfortunately, the magnesium oxychloride cement bond is relatively brittle, with a low coefficient of thermal expansion, and the strength of this bond can rapidly deteriorate under the heat generated during dry grinding. The cause of this is not clear; dehydration of the magnesite bond at elevated temperatures may be at least partially responsible. Whatever the etiology of this phenomenon, cracking of the bonding cement during grinding may occur, particularly after prolonged constant use. Such cracking is often followed by premature disintegration of the wheel.
2. Discussion of Related Art
Traditionally, liquid coolants, often fortified with lubricant material, are employed to dissipate heat engendered in various other grinding processes to protect both the abrasive tool and brittle or otherwise susceptible substrates from heat damage. Wet grinding applications employing liquid coolants are not, however, suitable for use in conjunction with magnesium oxychloride cement-bonded abrasive tools. Liquid coolants used to dissipate heat build-up during grinding with abrasive tools not based upon magnesite cement nearly always contain water. Since water softens and dissolves the magnesium oxychloride bond and causes rapid deterioration of the tool, these coolants cannot be used in conjunction with magnesite cement-bonded abrasive tools. Further, certain industrially-important substrates, notably the above-mentioned coil springs, do not lend themselves to wet grinding processes, and so such substrates are commonly dry-ground. As a result, magnesium oxychloride cement-bonded abrasive wheels or other abrasive tools are used almost exclusively for dry grinding applications, particularly for dry-grinding of water-sensitive substrate material such as coil springs, and the tools are at risk of developing deep cracks under the heat generated during grinding, followed by disintegration of the tool.
The incorporation of a wide variety of materials into cementitious compositions to reinforce the cured bond is known. However, few of these materials have proved useful in magnesium oxychloride cement bonds, for good reason.
It has been known for the decades that magnesium oxychloride cement has been in use as a bonding agent for abrasives in grinding applications that the material is highly sensitive to the incorporation of extraneous materials, which in additive-effective amounts typically have been found to weaken bonding of the cement precursors to the point of providing a commercially useless product. As noted above, the chemistry of the magnesium oxychloride cement bonding mechanism is not clearly understood, and addition of extraneous materials to the basic combination of calcined magnesium oxide, magnesium chloride, and water with a view toward improving the formulation has of necessity proceeded on an ad hoc basis. Many reinforcing materials have been suggested for strengthening hydraulic cement bonds other than magnesite bonds such as those based on Portland cement, as well as vitrified bonds and resinous bonds such as phenol formaldehyde-based resins to promote structural strength of the product. However, in combination with the present magnesium oxychloride cement precursors, such additives have typically proved to be deleterious to the cured bond, often weakening the bond structure of the product to the point of uselessness for the intended- application. Bonds exposed to unusual stress in use, most especially abrasive tools for dry-grinding applications, are particularly sensitive to added materials. Thus, the use of materials and processes known in the prior art for reinforcing magnesite bonds has been generally limited to bonds subjected to little or no stress. Construction materials based on reinforced magnesite cements as described in U.S. Pat. Nos. 3,320,077 to Prior et al.; 3,607,825 to Shannon; 4,033,752 to Isohata et al.; and 5,049,197 to Leroux et al. and are exemplary.
Theoretically useful reinforcing materials for obviating cracking and disintegration of magnesite cement bonded abrasive tools such as wheels have often proved disadvantageous in practice. One proposed reinforcing material for magnesite bonds is wire mesh, which has been incorporated into the bond to bridge across the matrix and resist cracking of the product under stress. However, if the product is to be used in abrasive applications, for example an abrasive wheel, the wire mesh cannot be present on a grinding surface of the wheel as wire of sufficient gauge to function as an effective reinforcement will interfere with the grinding process. Since the large majority of the wheel is consumed in grinding, the effectiveness of the wire mesh as a reinforcing agent in abrasive tools is limited. Other known reinforcing materials have proved to seriously compromise the heat strength of magnesite bonds, leading to cracking and disintegration of the bond when exposed to heat, such as in dry grinding applications.